This invention relates to electron beam deflection yokes for cathode ray tubes and more specifically to a single pair of coils for controlling the electron optics path of the electron beam of a shaped beam cathode ray tube. U.S. Pat. No. 3,473,077, by the same inventor and the same assignee of the instant invention discloses the state of the art in shaped beam cathode ray tubes and the state of the art means for electron beam control for character selection from a plurality of shaped apertures in a stencil through which the beam is extruded. The disclosed means is by the use of three separate pairs of coils whose composite action is such that the electron beam is directed from its axial path through a shaping aperture and then returned to an extension of its original path. The pairs of coils which are referred to as the selection convergence and reference yokes either have separate two axis deflection power supplies or are connected in series using a single two axis deflection power supply.
Deflection windings consist, generally, of paraxial segments which create a useful deflection field, and undesirable but necessary arcuate end connections which have components that, being normal to the beam direction, do not contribute to deflection but instead create distortive influences. The coils for deflection in a specified direction are wound in such a way that for each wire component, there is a diametrically opposite component of wire such that the resultant effect of the components at an axial point is supplemental with respect to the component of flux that will create the desired deflection but is a minimum for the unused components along the other two orthogonal axis. For off-axis points this cancellation of undesirable deflection effects becomes less and less effective as the off-axis distance increases. The distortion influence of the undesirable components of the winding normal to the beam axis limits the ability of the conventional three yoke system to perform shaped beam character selection in the regions intermediate to the selection and convergence yokes and to the convergence and reference yokes where the beam is at its maximum off-axis excursion and thus is most subject to the aforesaid limitations. The end turn effects of commercially available deflection yokes are usually reduced by shaping or bending up the end turns. This practice, however, is not consistent with the efficient use of wire for providing maximum deflection efficiency.
The use of three separate pairs of coils, each pair similar to the conventional two axis deflection yoke requires more power to operation, more material for construction, requires a more complicated alignment procedure for proper character selection and has more susceptibility to failure then a single pair of coils.